METAMORPHIC ROCK Chapter 6 Section 4

Objectives Describe the process of metamorphism. Explain the difference between regional and contact metamorphism. Distinguish between foliated and nonfoliated metamorphic rocks, and give an example of each.

Metamorphism Means "changed in form.” The process by which heat, pressure, and chemical processes change one type of rock into another.

Formation of Metamorphic Rocks Minerals may change in size and shape or separate into parallel bands to give a layered appearance.

Chemical Processes Change the mineral composition by dissolving some minerals and adding others.

The Type of Rock Formed Indicates the conditions that were in place when the original rock changed.

Two Main Types of Metamorphism

Clinker beds, eastern Montana Contact Metamorphism Rock which is metamorphosed because it is near or actually touching hot magma (or another heat source). Hot fluids also cause changes in the surrounding rock. Generally small in scale. Clinker beds, eastern Montana

Regional Metamorphism Metamorphism which is the result of heat and pressure caused by movement of tectonic plates. Large in scale.

Combinations Since volcanism often accompanies tectonic activity, rocks from contact metamorphism can be found within areas of regional metamorphism.

Classification of Metamorphic Rocks Can be classified by chemical composition, but, for first approximations, they are classified according to their texture (structure).

1st Major Type - Foliated Rocks Foliated rocks are layered, or they have visible parallel planes or bands of minerals.

Formation of Foliated Rocks Formed by extreme pressure forcing the minerals into parallel bands, or the minerals separate because they have different compositions and densities.

Common Metamorphic Rocks With Foliated Texture

Slate Forms from shale, which consists of clay minerals. The shale is compressed into even thinner layers. A hard rock that splits easily into flat sheets.

Schist With more pressure, schist forms from slate because the minerals grow into coarser (larger) grains.

Gneiss (nice) Forms under intense heat and pressure, causing the minerals to separate into bands of light and dark minerals.

2nd Major Type - Nonfoliated Rocks Nonfoliated metamorphic rocks are nonlayered and are without visible bands of aligned minerals.

They Reflect The Rocks They Were Formed From in 1 of 2 Ways The original rock was composed mainly of one type of mineral. The original mineral grains were round or square, so pressure did not change their position.

Common Nonfoliated Metamorphic Rocks

Quartzite Forms from sandstone, the original spaces between the particles disappear and may be filled with silica. Thus quartzite is very hard and durable.

Marble Forms from limestone, chemically reactive fluids can result in many different colors. Can be fine or coarse grained. May or may not effervesce (bubble) with acid (grind it up and it will).

Anthracite (hard) coal Forms from bituminous (soft) coal. THIS IS NOT IN YOUR BOOK, but to my mind and training (Mr. H), this is a metamorphic rock. Almost pure carbon.

Assignment – Due Friday 6.4 Key Terms Directed Reading Oh no, not again!